Radioreceiver



L. E. BARTON Jan. 28, 1936.

RADIORECEIVER Filed NOV. 25, 1933 3 Sheets-Sheet 1 D v, m Nh wmb E INvEiv-ron v L. E. BARTON A RADIOREGEIVER Filed Nov. 25, 1935 POWER SUPPL Y 3 Sheets-Sheet 2 Audio Fre q. Amp.

Control Gri Volts Filament Valt-s= 2.5

Screen. Grid Volts: 100

SuPPreSsor Volts O 4 2.o a- .f ,2.5 I/ 3,0 a-f 1| 4.0 I' 5.o o 16o zo ao 46o sbo PLATE voLTs INVEN TOR Loy E. Bart on/- Jan. 28, 1936. l.. E. BARTON RADIORECEIVER Filed Nov. 25,v 11953 3 Sheets-Sheet 3 QNVI I IN feu-ron Loy E.Ba,rton

HHH-BY ATTORN Y Patented Jan. 28, 1936 PATENT OFFICE RADIORECEIVER Loy E. Barton,

Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 25, 1933, Serial No. 699,641

13 Claims.

My invention relates to radio receivers and more particularly to receivers which are provided with noise suppressor circuits for preventing background noise from being heard when tuning between stations.

in the loud speaker when tuning from one station to another.

I-Ieretofore, noise suppressor circuits have generally required the use of a' special automatic volume control network including several additional ampliiier tubes. Circuits of this type increase the cost of a receiver a substantial amount. Also, it has been found somewhat diiiicult and expensive to so design such noise suppressor circuits that the noise suppression is complete.

An object of my invention is to provide a radio receiver having a noise suppressor which does not involve the above-mentioned disadvantages.

A further object of my invention is to improve the apparent selectivity of a radioreceiver, whereby the receiver must be accurately tuned to a signal before the signal can be heard.

In one embodiment of my invention I provide one of the audio frequency amplifier tubes with a self-biasing resistor in the form of the cathode-anode circuit of an electric discharge device such as a vacuum tube. The cathode-anode circuit or plate impedance of the vacuum tube is maintained at a high resistance value when no signals are being received, whereby the audio frequency amplifier is blocked. Upon the reception of an incoming signal, the plate impedance of the biasing tube is reduced in value sufiiciently to unblock the audiofrequency amplifier so that the signal can be heard. Preferably, an amplifier having a trigger action is utilized for controlling the plate impedance of the selfbiasing tube, in order that variations in said plate impedance shall be prevented during the reception of signals.

Other objects, features and advantages of my invention will appear from the following description taken in connection with the accompanying drawings in which Fig. 1 is a circuit diagram of a radio receiver l embodying one form of my invention;

(Cl. Z50-20) Fig. 2 is a set of curves showing certain characteristics of the self-biasing vacuum tube utilized in the circuit shown in Figfl and Figs. 3 and 4 are circuit diagrams showing other embodiments of my invention.

Referring to Fig. 1, an embodiment of my invention is shown applied to a superheterodyne receiver which comprises a radio frequency amplier I having an input circuit coupled to an antenna 3 through a radio frequency transformer 5. The output circuit of the amplifier I is coupled to the input circuit of a first detector 'I through a radio frequency transformer 9.

The secondary windings of the transformers 5 and 9 have tuning condensers II and I3, respectively, connected thereacross in the usual manner, for tuning the input circuits of the amplier I and the rst detector 'l to the desired incoming signal.

An oscillator I5 is coupled to thefirst detector l for the purpose of heterodyningan incoming signal to a lower intermediate frequency signal. The frequency of the oscillator output may be varied by means of a variable tuning condenser I'I.

The tuning condensers II, I3, and I 1, are preferably units of a gang condenser, and may be varied simultaneously through a common tuning control indicated by the dotted line I9.

The intermediate frequency output of the first detector 'l is impressed upon the input circuit of an intermediate frequency amplifier 2I through an intermediate frequency transformer 23, the primary and secondary of which are tuned to make the transformer function as a band-pass filter having a pass range of sufficient Width to pass the intermediate frequency carrier and at least one side band.

It will be understood, of course, that the tuned radio frequency circuits of the amplifier I and detector 'l are also tuned broadly enough to pass the carrier of the incoming radio signal and at least one of its side bands.

The output of the intermediate frequency amplifier 2| is fed to the second detector 25 through an intermediate frequency transformer 21 tuned in the same manner as the preceding transformer 23.

In the particular embodiment illustrated, the second detector 25 is a diode rectifier which is one unit of a double diode-triode vacuum tube 29, this rectifier having a plate 3I and a cathode 33.

One terminal of the secondary of the transformer 21 is connected to the detector plate 3|, while the other terminal of the secondary is con- V Y kwhich, in the embodiment shown, includes a nected to the cathode 33 through a resistor S35, the resistor 35 being shunted by means of an intermediate frequency bypass condenser 31.

The triode section 38 of the vacuum tube 29, which section functions as the Vfirst audio frequency amplifier, includes thevcathode 33, a control grid 39'and an anode 4|. The control grid 39 is connected tothe plate end of the resistor 35, whereby any audio frequency signals appearing across resistor 35 are impressed across the input of the triode section 33 to cause amplified audio frequency signals to appear in the plate circuit thereof.

Theiamplied audiofrequency signal is impressed upon the input circuit of another audio frequency amplifier comprising an electric discharge device such as a vacuum tube 43. In one embodiment of the invention, the Ytube 43 is an RCA 56. The signal is impressed upon the input circuit of tube 43 through a coupling system coupling resistor 45 in the plate circuitY of the triode section38.V The lower end of thecoupling resistor 45 is connected tothe positive terminal of a voltage divider 41 whichlis connected across any `suitable D. C. power supply 49. Voltage dvider 41 is connected to ground at a point inter- Y vmediate the two terminals thereof.

The upper end of the coupling resistor 45 ls coupled to a volume control resistor 5| through Y a coupling condenser 53, the lower end of the resistor 5| being connected to ground.

The control grid 55 of the amplifier 43 is connected toa point o-n the volume control resistor 5| through a variable volume-control tap 51.

Y AThe output of the amplifier 43 may befurther amplified if desirable by means of any suitable audiofrequency amplifier indicated at 5'9- and supplied to a loud speaker 6|. n

In accordance with my invention, the input circuit ofthe audiofrequency amplifier tube 43 includesY an electric discharge device 63 which preferably is ra'vacuum tube of the pentode type such as an RCA 57. 'Ihe cathode-anode circuit of the tube 63 is connected between the cathode 65 ofthe amplifier tube 43 and ground to function as a self-biasing' resistor which may be given a very high resistance to block the Vamplier tubev 43 when tuning betweenr stations. Connected in this way, the cathode-anode circuit of thetube 63 Y is common to the input and output circuitsr of amplifier 43 in the same manner as an ordinary Y V.selfbiasingresistor;Y An audio frequency bypass condenser'it` is connected between thecathode 61 andthe anode 69 of tube 63 in order to provide a low impedancerconnection from the grounded end of resistor 5| tothe cathode 65.A l Y The self-biasing tube A63 includesV inV addition to thecathode 61 and anode'69, a control grid 1|, a

screenrgrid 13, and a suppressor grid 15. In

operation, the plate impedance of the tube |53V is changed from a very high resistance value to a comparatively low resistance value by changing the potential on the control grid 1| from a high Y negative value to nearly a Zero value.

' A-resistor 16 of high resistance value is preferably connected between the plate 69 and the posi- Y tiveterminalof voltage'divider 41 for supplying part of the'plateland leakage current for the tube 63. The resistor 15 may be given a value of from f one-fourth megohm toone megohm, the value not being critical since 'the plate impedance of amplifier tube 43 is, so low Vcompared with` the im` pedance of resistor 16 that it, rather than resistor' 16, determines the voltage drop between the volt- Y L age divider 41 and the plate 69 of tube 63.

One function of resistor 16 is to keep the plate Yvoltage of the biasing tube 63 high during the period that the ampliertube 43 is blocked; The

general effect obtained by adding resistor` 16 toY the circuit is a more rapidand positive action Y of the noise suppressor. Y The control circuit for the self-biasing tube 63 includes a potentiometer 11 which is connected at one end to the negative terminal of the voltage divider 41,'and at therother end to the plate end ofthe coupling resistor 45. includes two resistors 19 and SI each of which may have a resistance value of approximately 1 megohmi f n i f The control grid 1| of biasing tube 53` is connected to a point on the potentiometer between the tworesistors 19 and 8| through a filter resistor V83 which may have a resistance value of approximately onemegohrn .1 An audio frequency The potentiometer l Y bypass condenser 85y is connected vbetween the control grid 1| and the cathode 61 for preventing audio frequency signals from Ybeing impressed Vupon control grid 1| 4and therebyvarying the plateirnpedance of the biasing tube during the f reception of signals.

The point on the potentiometer V11 to which the control grid 1| is connected, which is identified asV VVpoint B on the drawings, Vis preferably so chosen that it is highly: negative with respect to ground when there is no incoming signal.. For example, the point B may be at -20 Vvolts with respect to ground,.which is sufficient tol blockthebiasingtube 63, therebyY causing its plate impedance to be so high that theraudio frequencyfamplifier '43 is blocked. A Y

' When there'is no incoming signal, the high neg-V ative voltage is maintained on the control grid of the biasing'tube .63 in the following manner: With no incomingV signal, the control grid 39 of the rst audiofrequency amplifier 38 is at substantially the same potential as the cathode sincev the current flow through resistor 35 is small. Therefore, the plate current owing through the coupling resistor 45 has a fairly high value, and the upper end of resistor 4,5 is at a low positive potential. AsV previously stated, the point VB is so chosen that under these conditions, it is highly negative with respect to ground.

When arsignal is tuned in, the control grid 3'9 r of amplifier 38 becomesmore negative, and the the selfbiasing tube 63, when of the pentode or l suppressor grid type, will have a constant plate impedanceV over a fairly Wide variation in'c'ontroi gridvoltage- Y Obviously, however, the voltage applied to the control grid 1| 4must be held within reasonable limits during the reception of agsignal or the plate impedance Vof theA tube 63 will vary and cause distortion of thejsignal. j

In order to prevent too'great variations in the control grid ,voltageY and resulting signal distor- Y tion, I provide the receiverV withV an automaticY volume control circuit and trol device.

While various forms of automatic volume control may be utilized, the circuit illustrated in the drawings is simple and eective in operation. It includes one ofthe diode rectiiiers 81 of the vacuum tube 29, this diode comprising the cathode 33 and a plate 89. The plate 89 is connected through a iilter resistor 9| and a volume control resistor 93 to ground. Resistor 93 may have a resistance of the order of 500,000 ohms.

The junction point of the resistors 9| and 93 is connected to the control grids of the tubes 1, and 2| through resistors 95, 91, and 99, respectively. A filter condenser |0| is connected between the grid end of the lter resistor 9| and ground.

The cathode 33 is connected to the negative end of the voltage divider 41 through an automatic volume control resistor |03. This resistor, which is bypassed by means of an audiofrequency bypass condenser |05, may have a resistance of the order of 40,000 ohms.

The operation of the automatic volume control circuit is as follows: Assuming that a strong signal is being received, there is then maximum iiow of current through the second detector resistor 35, and the control grid 39 is at a rather high negative potential, whereby the plate current of triode section 38 is small and there is a small voltage drop in the volume control resisto-r |03. Therefore, the greater part of the voltage across the lower section `of the voltage divider 41 is impressed upon the diode 81.

This will be seen by noting that the grounded point of the voltage divider 41 is connected to the plate 89 of the diode rectifier 81 through a connection which may be traced from ground and through the volume control resistor 93, this grounded point being positive with respect to the cathode 33. The voltage across this lower section of 41 the voltage divider is large compared with the voltage drop in the volume control resistor |03 under the conditions assumed, so that although the two voltages are in opposition, the voltage applied to rectier 81 is only slightly less than the voltage across said lower section.

This results in a comparatively large iiow of current through the automatic volume control resistor 93, with the result that the voltage drop in resistor 93 maintains the control grids of the tubes l, 1, and 2| highly negative with respect to their cathodes, whereby their gain is held at a low value during the reception of the strong signal.

If the strength of the received signal becomes less, the control grid 39 becomes less negative, the plate current and the voltage drop in the resistor |03 increase, and a lower voltage is impressed across the rectier 81. As a result, the current flow through the volume control resistor 93 decreases and the control grids of the tubes 1, and 2|, become less negative, whereby the gain of the receiver is increased to bring up the signal strength. The above described automatic volume control circuit is described and claimed in my copending application Serial No. 640,946, iiled November 3, 1932, and assigned to the same assignee as this application.

In order to provide sensitivity control for the receiver the self-biasing resistors of tubes I, 1, and 2| may be made variable and connected to- With a sensitivity congether through a common sensitivity control in' dicated by the dotted line |01. By adjusting the sensitivity control, the point at which the automatic volume control circuit will start to function may be determined. Thus the sensitivity control may be so adjusted that only signals will be received which have a signal strength suiiicient to reduce the bias on the control grid 1| of the selibiasing tube 63 to a certain predetermined value, such as a value of approximately -3 volts. In the specific case where the self-biasing tube 53 is of the type known as RCA 57, this will prevent the receiverY from operating under conditions where the plate impedance of the biasing tube changes during the reception of a signal.

This feature will be more fully understood by referring to Fig. 2, which shows a family of plate curves for the RCA 57 tube. It will be seen that the plate curves for this tube, as well as for other tubes of the same general type, are concave downward. The most important fact to be noted from these curves is that the plate impedance of the RCA 57 tube is substantially constant for control grid voltages between zero and 3 volts when the plate voltage is very low. This fact is indicated by the dotted line |09 which shows how the plate curves all fall along a straight line at a low value of plate voltage for control grid voltages between zero and volts. It will be seen that as the control grid becomes more negative than -3 volts, the curves no longer fall along the line |09, showing that the plate impedance of the tube changes with a change in the control grid voltage at the more negative grid bias.

If the receiver were permitted to operate with the control grid 1| of the biasing tube 63 at approximately -5 volts, for example, the plate impedance would vary and the operation of the receiver would not be satisfactory. It will be seen, therefore, that by properly adjusting the sensitivity control, the A. V. C. circuit will cause the noise suppressor to operate Under conditions which Will produce an undistorted signal.

It has been pointed out above how the selfbiasing tube 63 is prevented from operating with its control grid too negative. It is prevented from operating with its control grid positive more than a slight amount above zero voltage by means of the filter resistor which limits the positive voltage on the control grid as soon as grid current begins to flow.

As an inspection of Fig. 2 Will indicate, the RCA 57 tube 63 should be operated at a plate voltage greatly below normal and at a grid bias Which is almost zero. In one embodiment of the invention where the tube 63 is an RCA 57 and the tube 43 an RCA 56, the voltage on the plate of tube 63 is 13 volts, the screen grid voltage of tube B3 is about 90 volts, and the plate current in the two tubes is about 4 milliamperes. The grid bias of tube 43 is normally 13 volts while its cut-oli bias is approximately 20 volts, this is assuming a normal plate voltage of 250 volts and a drop in plate voltage to about 170 volts as the tube is'biased to cut-off.

It should be noted that tube 43 is biased to cut-olf very rapidly since its plate voltage is reduced in value as the biasing voltage is increased. The above-mentioned plate voltage reduction to 170 volts is caused by an 80 volt drop across the tube 63 produced when it is biased substantially to cut-olf during the period that no signal is being received. It will be seen that the volt drop is much greater than the bias voltage required for biasing tube 43 to cut-off, whereby the noise suppression is complete.

If desired, the receiver shown in Fig. l may be operated without noise suppression by closing the switch ma, Yuns connecting the contrai gridV 1|,`

. biasing tube 63 is connected to the upper end of the coupling resistor 45. The Voltage divider 41 and theV coupling resistor 45 are so adjusted that Vwhen there is no incoming signal, the control grid 1| is sufficiently negative with respect toits cathode 51 to block the tube 63,'as described in connection with Fig. l. this adjustment can be made readily by grounding the voltage divider r41Vat a point suchthat the voltage drop inthe coupling resistor 45 will be greater than the 'voltage'across the upper section of the voltage divider. LBy employingY this circuit, the noise suppressor may be made torespond to an incoming signal somewhat more rapidly than when employing the circuit shown l rbecause the absence of the potentiom- Fig.4

in the grid circuit of the biasing eter 11 results .tube 53 having a shorter time constant.

In the embodiment of my Vinvention shown in Fig. i, there Yis an additional control circuit for the noise suppressor, this control circuit being provided both fork the purpose of increasing the effective or apparent selectivity of the receiver and for preventing operation of the self-biasing tube 63 under conditionsrthat might produce signal distortion. In Figsl and 4, like parts are indicated by the same reference numerals.

Referring to Fig. 4, tineY coupling circuit be- V tween the rst audio frequency amplifier 38 'and ably includes a choke coil An audio frequency bypassv the second audio frequency amplier 43 prefery I inthe plate circuit of the amplifier 38.

Vcondenser ||3 is connected between the lower above-mentioned control circuit.

end of the choke coil andthe cathode 33. The lower end of the choke coil is connected to the positive terminal of the voltage divider 41 through a conductor ||5.

As in the circuit shown vin Fig. l, the control grid 1! of the biasing Ytube 63 isconnected to a potentiometer eter 11 is connected between the negative terminal'of the voltage rdivider i1 and the upperV endV of a plate Vresistor l i1 which is included in the plate circuit of a control tube |i9 in the The control Vtube VH9 is preferably a diodepentode vacuum tube having diode section and a Vpentode section '|23.- The pentode secktion includes a cathode |25, control grid |721,V Va screen grid |29, a suppressor grid' |3|, and an anode |33. The diode section includes the cath- Y ode |25 and a plate |35. l Y

' Therinput circuit ofthe pentode section |23 is coupled tothe second detector resistor 35 in any suitable manner for impressing the low intermediate frequency voltage drop thereof across the pentoderamplier input electrodes. InY the drawings, this coupling device is shown asa small coupling condenser |31, having acomparatively low impedance to signals at the intermediate frequency, but having'an .impedance to audio frequency signals suicient to prevent any appreciarnplifier tube 29 is connected tother ItV will be noted thatV 11, but in this case the potentiom- Y Y of the diode' circuit.

able audio frequency 'voltage fromV being im; pressed upon the input circuit of pentode sectionlZS. f

A resistor |391is provided which is connected at one end tothe control grid |21 through a conf ductor |4| ,and connected atV the other end to the cathode |25 through an audio frequencybypass condenser |43. The resistor |39 is Vshunted by a variable condenser |45 which may beV adjusted for the purpose of controlling the input voltage to the pentode amplier |23. o i

The plate circuit of the'pentode vsection |23 includes the primary winding |41 of a transY formerV |49 and the resistor !|1, the 'primary winding and the resistor being connected in se-V ries, and the lower end ofV the 'resistor being connected to a positive Vpoint on the Voltage vdivider 41'through a conductor |5|. i'

The secondary winding |53 of the transformer |49 is loosely coupled to the primary vwinding |41,

and both theiprimary and the secondaryv are-V tuned so that the transformer is tuned very sharply to the intermediate frequency. Thus, the

tuning of the noise suppressor circuit is much sharper than the' tuningrof thesignal channel.

A,The amplified intermediate frequency output of the'pentode-section |23'is transferredthrougn the sharply tuned transformer |49 and impressed upon the elements |25 and |35 of the diode rectifier through a conductor |55 (which connects the diode plate |35 to one terminal of the secondary winding |53) .and through VaV connection 'Y Y from the cathode |25 to the other terminal of` the secondary |53, which connection may be traced from the cathode |25 throughja self-biasing resistor |51 to ground, through ground 'to VVthe lower terminal cfa resistor |59, and through` the resistor|59 to the said other terminal of secondary |53. 'Resistor |59 may be given a value ofthe order of 1/2 megohm. e' Y The function of the diode rectifier |2| Vis'to increase the negative bias on the control grid |21 of the pentode section upon reception of a radio signal. Since a flow of current through the rectier |2| causes the upper end of the resistor |59 to become negative, the control'grid'i21 also becomes more negative because of its connection to the negative end of the resistor 59 through the resistor |39 and ahighpimpedance resistorY |6l. Resistor |6| may'havea resistance value of therorderof 1/2 megohrn.' Y Y Obviously an Aaudio frequency signal will appear in the circuitV of rectifier |2|A which should not be impressed upon the control grid |21 of the pentode section.' This is prevented by the audio frequency bypass condenser` |43. The resistor |59, the selfbiasing resistor |51 and the plate resistorV |1are shunted by the usual intermedi-Y ate frequency VbypassV condensers |53, Y|65 and |91, respectively. f' i It will be apparent from the foregoingrdescri'p- (tion that as soon as an intermediate frequency signal is impressed upon Vthe input circuit oftheV Y Vpentode amplifierv |23, the controlV grid |21 of that amplifier Vwill become more negative because of y the current iiow through the circuit of the recti- It maybe noted that the change inr-biasnonY the control grid |21 depends; to a certain extent, f

'Ihat` these Vvoltage drops are in series will be made evident by tracing the direct 'current path ofthe", pentode section input circuit. It may be traced from the control grid |21 through the coupling resistor |39, the high impedance resistor |6I, through the diode circuit resistor |59 to ground, through ground to the self-biasing resistor |51, and through the selfbiasing resistor |51 to cathode |25.

When there is no incoming signal, the plate current of the pentode amplifier |23 is large, since the only negative bias on the control grid |21 is that due to the self-biasing resistor |51, and this negative bias is comparatively small. Consequently there is a large voltage drop in the plate resistor ||1, whereby the upper end of resistor ||1 is at a fairly highnegative potential with respect to the positive end of the voltage divider 41. Also, the gain of the amplifier |23 is low because of the low voltage of its anode |33.

As soon as the receiver is tuned to an incoming carrier Wave, the amplifier |23 supplies signal energy to the diode |2|, the Voltage drop in the diode circuit resistor |59 increases the negative bias on the control grid |21, whereby the plate current of amplifier |23 decreases and the upper end of the plate resistor ||1 becomes less negative and the voltage applied to anode |33 is increased. This increases the gain of amplier |23, and causes a further decrease in plate current through resistor I1, this action continuing until the plate voltage builds up to a maximum value. At this point the gain may have decreased because of relatively high bias. It is the potential at the upper end of the plate resistor ||1 which is utilized for actuating the noise suppressor.

It should be noted that although the decrease in plate current tends to cause a reduction in the biasing voltage in the self -biasing resistor |51, the

increased negative bias provided by the current flow through the diode circuit resistorl |59 is much greater than any decrease in bias due to reduction in plate current.

The above described action of the diode-pentode circuit, whereby the upper end of the plate resistor ||1 becomes less negative in response to reception of a radio signal, is a trigger action which is obtained by giving plate resistor ||1 a proper resistance value. This value is not critical and may be between 50,000 and 100,000 ohms. This trigger action depends upon the fact that the increase in gain caused by the increase in the plate voltage is greater than the decrease in gain caused by the increase in negative bias on the grid |21 up to a certain bias after which the gain decreases. In the particular circuit described above, an RCA 2B7 tube has been employed for the tube I9l with satisfactory results.

It may be noted that the gain of the usual threeelement vacuum tube remains substantially constant over a wide variation in plate voltage so that it is not suitable for providing a trigger action in my control circuit. It should be understood, however, that it is not essential that the amplier |23 be adjusted to operate as a trigger amplifier, although, for reasons explained below, such an adjustment is highly desirable.

The trigger action adjustment is preferred as the control circuit then never permits the control grid 1| of the biasing tube 63 to remain at a voltage such that the tube operates with the plate impedance changing with comparatively small variations in grid voltage. As soon as a signal is received, the trigger action of the control circuit causes the bias on the control grid of the biasing tube to change abruptly from its high negative value of -20 volts, for example, to a value which is approximately zero, for example, in the region where the impedance of the plate circuit is constant during minor variations in control grid voltage.

The use of a resistor in the plate circuit of a pentode for providing a trigger amplifier is described and claimed in my 'co-aending application Serial No. 704,510, filed December 29, 1933 and assigned to the same assignee as this application.

It Will be seen that the apparent selectivity of the receiver illustrated in 4 is determined largely by the sharpness of the tuning of the transformer ld. Unless the receiver is tuned eX- actly to the carrier frequency of a signal, the amplier tube 43 will remain blocked, and the signal will not be heard.

It should be understood that while I have referred to speciiic types of tubes and specific impedance and voltage values, the invention is not limited to the use of such tubes and values. They have been referred to only for the purpose of malring my invention more readily understood.

Various modifications in addition to the ones described may be made in my invention without departing from the spirit and scope thereof and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior i art and set forth in the appended claims.

I claim as my invention:

1. In a radio receiver having a signal channel, an electric discharge tube in said signal channel, said tube having a cathode, an anode and a control grid, and a control tube having a cathode- 'anode space discharge path, said control tube being connected with said space discharge path common to the anode and control grid circuits of said electric discharge tube whereby it functions as a self-biasing resistor for said control grid.

2. In a radio receiver, an audiofrequency amplifier including an electric discharge device having a cathode, an anode, and a control electrode, an input circuit including said cathode and said control electrode, an output circuit including said cathode and said anode, and a second electric discharge device having a cathode-anode space discharge path common to said input circuit and said output circuit whereby it functions as a self-biasing resistor for said control electrode.

3. A radio receiver according to claim 2 characterized in that the receiver includes means for controlling the impedance of said space discharge path in response to an incoming radio signal.

4. In a radio receiver, a plurality of electric discharge devices connected in cascade, at least one of said devices having a cathode, an anode, and a control electrode, said one device having an input circuit including said cathode and said control electrode, said one device also having an output circuit including said cathode and said anode, and a second electric discharge device having a cathode-anode space discharge path common to said input circuit and said output circuit, and means for controlling the impedance of said space path in response to an incoming signal.

5. A radio receiver according to claim 4 characterized in that the second discharge device has a control electrode which is maintained at a negative potential sufficient to bias said second device to cut-olf when no incoming signal is being received. I

6. A radio receiver according to claim 4 characterized in that when no incoming signal is being received the impedance of said space path is suicient to bias said iirst discharge device t0 cut-off.

7. In combination, A.an electric discharge device having an input circuit and an output circuit, and a pentode having a cathode-anode space discharge path which is common to said input and output circuits whereby it functions as a self-biasing resistor for said device.

S. Apparatus according to claim 7 character- VYized in that means is provided for controlling the potential on the control grid of the pentode.

9. In combination, an electric discharge device having an input circuit and an output circuit, and a vacuurntube of the suppressor grid type having a cathode-anode spaceY discharge path which is common to Vsaid input and output circuits whereby it functions as a self-biasing resistor for said device.

K 10. In a radio receiver, a plurality of amplifiers connected in cascade, one of said ampliers including an electric discharge device having anV input circuit and an output circuit, a space discharge device having a space discharge path and a control electrode, said space discharge path being common to said input and output circuits, said space discharge device having the characteristic that for a certain range of potentials applied to said control electrode the impedance of said space discharge path remains at a substan-V tially constant and comparatively low value,

' means for normally maintaining said control electrode at a potential which causes the impedance of said space discharge path to be high, and means for changing the potentialof said control electrode to a value within said certain range in response to an incoming signal.

l1. A receiver according to claim 10 characterized in that said last means includes a trigger amplifier whereby said control grid potential is changed suddenly and positively to a value within said certain range in response to an incoming signal.

12. In combination, an electric discharge device having an output circuit and an Yinput circuit includingV a control electrode, and an electric discharge device having a cathode-anode space discharge path common to said input and output circuits whereby a voltage drop across said space discharge path is applied to said control electrode to supply a biasing potential thereto, said second device having the characteristicthat its plate current plotted against plate Volts is 

